1. Field of the Invention
The present invention relates to a smoke sensor and in particular to a smoke sensor which detects the density of smoke by emitting light and then receiving light reflected by the smoke or light which has passed through the smoke.
Furthermore, the present invention relates to electronic equipment having a smoke sensor and in particular to electronic equipment having a smoke sensor which detects the density of smoke by emitting light and then receiving light reflected by the smoke or light which has passed through the smoke.
The present invention also relates to a smoke sensor which is preferably mounted on an air cleaner, a fire alarm, a sprinkler, or the like.
2. Background Art
Conventionally, in a smoke sensor which detects the density of smoke by receiving light which has been emitted from a light source and then reflected by the smoke, because the quantity of the reflected light is very small, the output of a light-receiving element receiving the reflected light is very small. Therefore, the output of the light-receiving element is amplified by an amplifier circuit and it is determined that the smoke has been detected when the output of the amplifier circuit has reached a predetermined range.
However, the output changes affected by a change in the quantity of light emitted by an LED caused by a temperature change or deterioration or the like of the LED even if the density of smoke to be detected is in the same range. For this reason, there is a problem that smoke is detected outside a smoke density detection range which has been set. In order to overcome this problem, various correction methods have been proposed.
For example, it has been proposed that a thermistor is added to a light-emitting element driving circuit to make a temperature correction in order to correct a condition that the quantity of light emitted by a light-emitting element is reduced due to its temperature characteristic and the quantity of light reflected by smoke is thus reduced. In this connection, it has also been proposed that when an output changes due to the temperature characteristic of an IC, a thermistor is added to a resistor section deciding an amplification factor, and the amplification factor is changed by the thermistor to make a temperature correction when there is a temperature change.
Furthermore, it has also been proposed that a detector including a light-receiving element monitoring a reduction in the quantity of light emitted by an LED caused by its deterioration or the like is provided in addition to a detector including a light-receiving element detecting smoke and a change in an output signal of the light-receiving element monitoring the quantity of emitted light is fed back to a light-emitting element driving circuit to correct the quantity of emitted light.
Furthermore, it has also been proposed that in order to make a temperature correction, a light-receiving element detecting no smoke is provided in addition to a light-receiving element detecting smoke and the difference between the outputs of the two light-receiving elements is amplified to detect the difference between the outputs of the two light-receiving elements and thereby an output change caused by a temperature change is corrected.
The above conventional technologies have a problem that errors for which no cause is identified cannot be corrected, although an error for which a cause such as a temperature change or a change in the quantity of light of an LED caused by its deterioration is identified can be corrected.
For example, the technology which adds a thermistor to make a temperature correction is able to correct a temperature change but is not able to correct a change in the quantity of light emitted by an LED caused by its deterioration. This technology requires selection of a thermistor suitable for temperature correction and may not obtain a sufficient correction.
Furthermore, of the technologies which correct a reduction in the quantity of light emitted by an LED, the technology which uses a detector including a light-receiving element monitoring a reduction in the quantity of light emitted by the LED caused by its deterioration in addition to a detector including a light-receiving element detecting smoke, and feeds back a change in an output signal of the light-receiving element monitoring the quantity of emitted light to a light-emitting element driving circuit to correct the quantity of emitted light, is of course not able to correct for a temperature change, and what is worse is that the technology needs a significantly complicated configuration and an increased number of parts including the two detectors, etc.
Regarding the method, which provides a light-receiving element detecting no smoke in addition to a light-receiving element detecting smoke in order to make a temperature correction, amplifies the difference between outputs of the two light-receiving elements to detect a differential current between the two light-receiving elements, and thereby corrects an output change caused by a temperature change, this method has been conceived in a light extinction system in which the light-emitting element and the light-receiving elements are arranged facing each other.
In the light extinction type, the light-receiving element detecting no smoke also directly receives light, so that the two light-receiving elements have similar outputs under a condition of the same quantity of light of an LED and the same amplification factor at a normal time when there is no smoke, while when smoke has flowed into the smoke sensor, the differential current of the two light-receiving elements and hence the output of the amplifier increase and thereby the smoke can be detected. However, the light extinction type has a problem that the light-receiving element detecting smoke directly receives light from the light-emitting element when there is no smoke and the light-receiving element detecting no smoke always directly receives light from the light-emitting element, and therefore the light-receiving elements easily deteriorate.
On the other hand, in a reflection type, when there is no smoke, there is no light reflected by smoke and therefore a light-receiving element detecting smoke receives a small quantity of light and the output of it is small, while a light-receiving element detecting no smoke directly receives light and therefore the quantity of light received by it is large and the output of it is large. Thus, when the same quantity of light emitted by an LED and the same amplification factor are applied for each of the light-receiving elements, the output of the light-receiving element detecting no smoke becomes too large to be saturated and therefore a true difference may not be obtained.
The output difference obtained when there is no smoke is large. But, as the density of smoke increases, the output difference becomes smaller, and when the density reaches some value, the output difference becomes zero. If the density of smoke then further increases, the output difference increases again. Thus, depending on adjustment, the output difference obtained when there is no smoke may be the same as an output difference obtained when there is much smoke and therefore a correct detection may not be able to be done.
Furthermore, when the quantity of light emitted by an LED, an amplification factor, etc. are adjusted according to a light-receiving element detecting no smoke, an original smoke detection accuracy is reduced and therefore the density of smoke cannot be detected accurately.